XV11 



present views, we perceive at once what a start the French chemist 

 had gained on his contemporaries. " I am engaged in a series of 

 experiments," he says, "intended to fix the atomic weights of a con- 

 siderable number of bodies, by determining their density in the state 

 of gas or vapour. There remains in this case but one hypothesis to 

 be made, which is accepted by all physicists. It consists in sup- 

 posing that in all elastic fluids observed under the same conditions, 

 the molecules are placed at equal distances, i.e., that they are present 

 in them in equal numbers." 



It is obvious that the author opens his inquiry with the very con- 

 ceptions which form the basis of our present views in chemical philo- 

 sophy ; and it is only to be wondered at that the happy enlistment of 

 Avogadro's ideas into the service of chemistry, which we owe to 

 Dumas' initiatory sagacity, should for more than a quarter of a 

 century almost have fallen into oblivion. 



Having premised in lucid terms the general scope of his inquiry, 

 Dumas proceeds to describe the several modifications of the well- 

 known method of taking vapour-densities with which he has endowed 

 science, and which went forth from his hands in such a state of con- 

 summate perfection that there has scarcely been room left for subse- 

 quent emendation. 



The narrow compass of this sketch does not permit us to quote the 

 numerous results communicated by Dumas in his paper ; we will 

 mention, however, that the determination of the vapour-density of 

 chloride and fluoride of silicon elicited the view now held regarding 

 the constitution of silicic acid, thus overthrowing the old formula 

 of this compound upon which Berzelius had based his classification of 

 silicious minerals. 



There were other experimental researches of importance carried 

 out by Dumas about this period. It had long been his intention to 

 resume the study of the compound ethers, to which he had devoted 

 considerable attention at Geneva. In conjunction with Boullay he 

 proceeded to an elaborate investigation of nitrous, acetic, benzoic, and 

 oxalic ethers. The composition of these su bstances was finally settled 

 by accurate combustions and vapour- density determinations. They 

 further elicited by unequivocal experiments the capital fact that the 

 decomposition of compound ethers by alkalies gives rise to quantities 

 of acids and alcohol, the joint weight of which is greater than 

 the weight of the compound ethers submitted to experiment, and 

 by accurately determining this difference they succeeded for the first 

 time in establishing the nature of compound ethers on the solid 

 foundation of experiment. 



These investigations led to other inquiries teeming with most 

 remarkable results, among which, those on the formation of 

 oxamide, on chlorocarbonic ether, and on urethane may be specially 



